Method of quantitating milk protein mRNAs in mammary tissues and in cultured mammary explants was developed. This method has been used to evaluate the effect of insulin, hydrocortisone and prolactin on the production of alpha-lactalbumin, whey phosphoprotein and k-protein. Stimulatory effect on the production of alpha-lactalbumin and whey phosphoprotein mRNA can be seen in the presence of insulin and hydrocortisone alone. Although prolactin enhances this effect, the results indicate the hydrocortisone and insulin but not prolactin plays a key role in the regulation of alpha-lactalbumin and whey phosphoprotein mNRA levels.